As a traction control system for a vehicle which prevents deterioration of acceleration performance when a wheel skids during acceleration of the vehicle or the like, Tokkai Hei 2-63934 published by the Japanese Patent Office in 1990 discloses the concept of reducing the output power of the engine along with operating the brake when skidding of a vehicle driving wheel is detected.
Moreover, Tokkai Hei 5-155264 published by the Japanese Patent Office in 1993, for example, discloses a torque splitting control system which changes the apportionment of drive power between the front wheels and the rear wheels of a four wheel drive vehicle.
Although either of these prior art proposals offers a technique for preventing drive wheel skid, the following problem arises with a four wheel drive vehicle equipped both with a traction control system and with a torque splitting control system.
When for example a wheel of the vehicle spins freely upon a road whose coefficient of friction .mu. is low such as a road bearing an accumulation of snow, if the torque splitting control system operates before the traction control system, then a delay time period occurs before the traction control system operates, and the wheel is not prevented from skidding to some extent during this delay time period. While this free spinning is occurring the coefficient of friction .mu. of the road surface becomes yet lower, and as a result the starting off capability and the acceleration performance of the vehicle deteriorate.